In recent years, the conservation of energy in heating and cooling installations has become very important. The rapid rise of the cost of fuel has caused serious interest in the development of thermostatic control which is fuel efficient and which provides convenience. Convenience has been made available by the development of microcomputer or microprocessor operated thermostats. These thermostats utilize a microprocessor or microcomputer to program desired setpoint temperatures at specifically determined times. This allows for the control of temperature within a building or enclosed space in a very accurate and energy efficient manner.
The energy savings come about by the programming of an energy saving function. In a heating application the energy saving function is accomplished by lowering the setpoint during a time when the space is unoccupied or when the space can be utilized at a lower than normal temperature. The temperature can then be raised at a convenient time. Typically this can occur twice during a day in a normal residence. A first setback temperature is typically used during the nighttime hours to save energy while the occupants are asleep, and is again used during the daytime hours when the occupants of the building are away at work. This same function can be applied to cooling wherein the temperature of the building or space is raised during an unoccupied time or when the higher temperature can be comfortably adapted to. The temperature is then lowered when the building or space is normally occupied.
The heating and cooling setback and setup functions are well known, and have been developed to a high degree by the use of microcomputer or microprocessor based thermostats. These thermostats also have utilized a function known as optimum start. Optimum start is the concept of initializing the heating or cooling before a new setpoint change occurs in order to have the new setpoint change occur at about the same time the temperature actually reaches the newly designated setpoint. This type of control, however, is generally considered to be incompatible with a heating and cooling thermostat that utilizes automatic changeover.
Automatic changeover in a heating and cooling thermostat is accomplished by the thermostat automatically determining whether the heating plant or cooling plant should be in operation. This is normally accomplished by providing a fixed differential between the heating and cooling setpoint temperatures, and allowing the temperature changes to activate the correct temperature altering plant. The addition of optimum start to an automatic changeover thermostat can create problems. The optimum start may occur at a time prior to the correct setpoint for the thermostat, and can result in the heating and cooling plants operating alternately in an attempt to provide the desired control. This conflict between the heating and cooling plants must be eliminated to make a practical automatic changeover thermostat having optimum control.